1. Field of the Invention
The present invention relates to a twincam engine, and a motorcycle comprising the twincam engine. More particularly, the present invention relates to a construction for enabling an intake port and an exhaust port to have large curvature radiuses.
2. Description of the Related Art
For example, there is an inline four-cylinder four-cycle engine as an engine mounted in a motorcycle. Focusing attention on a valve system, there is a twincam engine (also referred to as DOHC (double overhead camshaft)) having rocker arms, as one type of the inline four-cylinder four-cycle engine. Such a twincam engine includes a camshaft configured to drive an exhaust valve and a camshaft configured to drive an intake valve, which are mounted between a cylinder head and a cylinder head cover and are disposed forward and rearward. These camshafts rotate in association with a crankshaft, and its rotation is transmitted through cams and the rocker arms to reciprocate the exhaust valve and the intake valve (see Publication of Japanese Examined Utility Model Application No. Hei. 3-15761).
A construction of the valve system will be described in detail. The exhaust valve and the intake valve are arranged forward and rearward for each cylinder, and their valve stems extend upward from their disc portions disposed at openings of the exhaust port and the intake port which are located on a combustion chamber side. A rocker arm on the exhaust valve side is disposed forward in the engine. A tip end portion of the rocker arm on the exhaust valve side is in contact with a valve lifter which is in contact with an upper end portion of the valve stem of the exhaust valve. The rocker arm is pivotally attached at a base end portion thereof to a cylinder head at a location forward of the valve stem by a pin. A rocker arm on the intake valve side is disposed rearward in the engine. A tip end portion of the rocker arm on the intake valve side is in contact with the valve lifter which is in contact with an upper end portion of a valve stem of the intake valve. The rocker arm is pivotally attached at a base end portion thereof to a cylinder head at a location behind the valve stem by a pin. The pin for mounting the rocker arm on the exhaust valve side extends in a rightward and leftward direction and is a common pin for the respective cylinders. The pin for mounting the rocker arm on the intake valve side is constructed in the same manner.
In the engine having the above constructed valve system, it is necessary to lay out the exhaust port and the intake port extending upward from the combustion chamber so as not to interfere with the pins located above the combustion chamber. For this reason, the exhaust port and the intake port must be respectively curved forward and rearward with relatively small curvature radiuses. If the curvature radius of the intake port is small, then it is difficult to improve air-intake efficiency. If the curvature radius of the exhaust port is small, then it is difficult to improve exhaust efficiency. This is undesirable to a high-output and high-speed engine. Such a circumstance occurs in twincam engines mounted in other vehicles as well as the engines mounted in the motorcycles.